1. Field of the Invention
This invention relates to the operation of chlor-alkali cells having diaphragms made of synthetic fiber and exhibiting adequate service life and adequate performance characteristics. It concerns, in particular, such diaphragms which give, in addition, improved performance during an initial period of operation of a chlor-alkali cell provided with such a diaphragm, and in one aspect, it relates to a method of renewing one cell unit in a group of series-connected cells in a cell room.
2. Description of the Prior Art
The above-mentioned copending application adequately describes the importance to the brine-electrolysis industry of a development whereby a synthetic fibrous material is used to replace asbestos, yet adequate service life and performance characteristics are obtained. It was found, however, that there was need for an improvement in respect to the performance of such synthetic-fiber diaphragms cells during an initial period of operation, of up to about 300 hours. When work was conducted upon laboratory-scale cells provided with such diaphragms, it was observed that during such initial period of operation, a cell voltage higher than desirable would be required in order to maintain a given current through the cell. This is not a serious drawback with respect to the operation in an individual laboratory-scale cell, because it would be possible either to accept during such initial period a lower rate of production of chlorine and caustic by using a lower current or to take measures to cause the liquid in the cell to be maintained at a temperature lower than it would be if no such measures were taken and a level of current usual for the production of chlorine and caustic at commercial production rates were used. The drawback of an initially high cell voltage, is, however, a serious problem with respect to the use of synthetic-fiber diaphragms of the kind mentioned in the above-mentioned copending application when it comes to the use of such synthetic-fiber diaphragms in cell units of a group of series-connected cells in a cell room, for reasons which will be explained in detail below.
Relatively high cell-voltage during an initial period of operation is a problem, in respect to commercial utilization of synthetic-fiber diaphragms, largely because it is essential to prevent the liquid in the cell diaphragm from boiling. If the liquid in the diaphragm boils, the diaphragm is likely to rupture and become substantially inoperative. A group of series-connected cell units is operated at some current such as 25,000 to 120,000 amperes, i.e., a current density on the order of 130 to 150 amperes per square foot; the current is necessarily the same through each cell unit in the series-connected group. If a given single cell diaphragm has a relatively greater resistance, i.e., a greater cell voltage, there is thus evolved within that particular cell unit a relatively greater amount of heat. This implies that it is necessary to keep the cell voltages of each of the members of the series-connected group relatively at about the same value.
Without the improvement of the present invention, it would be possible to start up an entire series-connected group of cell units provided with new synthetic-fiber diaphragms and operate for an initial period of time with a relatively low amperage, accepting for the first 10 days to 2 weeks of operation a relatively lower rate of production. It would also be possible, later on, when an individual cell unit requires replacement of its diaphragm, to replace it with one which has been operated separately under suitable conditions for a period of approximately 2 weeks, so that its cell-voltage characteristics would not differ too greatly from those of the others in the series-connected group of cell units. It is unavoidable, in the commercial context, that removal of individual cell units of the group will be required, because it is impossible to predict exactly how soon an individual cell unit will require renewal; it is not, moreover feasible to shut down the entire series because one or two of the units require renewal, and it is not desirable to allow an increasing number of individual units which have become unsatisfactory to be taken out of service and not renewed. Thus, there is need for an improvement such as that provided by the present invention.
The most pertinent prior art of which we are aware, apart from the above-mentioned application, is British Pat. No. 1,081,046, which teaches the use for the electrolysis of brine of a diaphragm made of polytetrafluoroethylene and describes, as an additional feature, the use of an inorganic filler material such as barium sulfate, titanium dioxide, or the amphibole or serpentine forms of asbestos. This British patent does not relate to diaphragms made of fluoro-carbon polymers in the form of an entanglement of very fine fibers, such as to produce the desired degree of permeability of the diaphragm; instead, the teachings of the British patent are concerned with the making of synthetic-material diaphragms wherein a different technique is used: polytetrafluorethylene in the form of an aqueous dispersion of sub-micron-sized particles is mixed with a "solid particulate additive", such as starch or calcium carbonate, which is substantially insoluble in the aqueous dispersion medium from which the diaphragm is formed but is capable of being removed from the sheet by treatment with hydrochloric acid or the like to form a diaphragm sheet of the desired porosity. Thus, the British patent does not begin to provide those skilled in the art with a technology based upon the use of a suitable fluoro-carbon polymer in the form of very fine fibers, so as to make it possible to replace asbestos completely and obtain satisfactory service life and operating characteristics; moreover, the British patent, insofar as it teaches the inclusion of inorganic materials in its diaphragms, only teaches the use of this feature for extending the operating life of the diaphragm and better maintaining the permeability of the diaphragm while it is in use, and it gives no indication of the connection between the use of such inorganic materials and the initial improvement in cell-voltage characteristics which the applicants have observed.